On Fri, 19 Aug 2011 14:14:11 -0500
Andrew Barry [off-list ref] wrote:

This patch fixes a use-after-free problem in free_huge_page, with a quota update
happening after hugetlbfs umount. The problem results when a device driver,
which has mapped a hugepage, does a put_page. Put_page, calls free_huge_page,
which does a hugetlb_put_quota. As written, hugetlb_put_quota takes an
address_space struct pointer "mapping" as an argument. If the put_page occurs
after the hugetlbfs filesystem is unmounted, mapping points to freed memory.

OK.  This sounds screwed up.  If a device driver is currently using a
page from a hugetlbfs file then the unmount shouldn't have succeeded in
the first place!

Or is it the case that the device driver got a reference to the page by
other means, bypassing hugetlbfs?  And there's undesirable/incorrect
interaction between the non-hugetlbfs operation and hugetlbfs?

Or something else?

<starts reading the mailing list>

OK, important missing information from the above is that the driver got
at this page via get_user_pages() and happened to stumble across a
hugetlbfs page.  So it's indeed an incorrect interaction between a
non-hugetlbfs operation and hugetlbfs.

What's different about hugetlbfs?  Why don't other filesystems hit this?

<investigates further>

OK so the incorrect interaction happened in free_huge_page(), which is
called via the compound page destructor (this dtor is "what's different
about hugetlbfs").   What is incorrect about this is

a) that we're doing fs operations in response to a
   get_user_pages()/put_page() operation which has *nothing* to do with
   filesystems!

b) that we continue to try to do that fs operation against an fs
   which was unmounted and freed three days ago. duh.


So I hereby pronounce that

a) It was wrong to manipulate hugetlbfs quotas within
   free_huge_page().  Because free_huge_page() is a low-level
   page-management function which shouldn't know about one of its
   specific clients (in this case, hugetlbfs).

   In fact it's wrong for there to be *any* mention of hugetlbfs
   within hugetlb.c.

b) I shouldn't have merged that hugetlbfs quota code.  whodidthat. 
   Mel, Adam, Dave, at least...

c) The proper fix here is to get that hugetlbfs quota code out of
   free_huge_page() and do it all where it belongs: within hugetlbfs
   code.


Regular filesystems don't need to diddle quota counts within
page_cache_release().  Why should hugetlbfs need to?

...

+		/*Free only if used quota is zero. */

Missing a space there.

The above need documenting, please.  That documentation would perhaps
help me understand why we need both an "active" flag *and* a refcount.

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